Generally, sintered silicon nitride has been manufactured in batch operations by first placing a silicon nitride article in a refractory material container, formed typically of graphite. A setter powder of silicon nitride or boron nitride or a mixture thereof is then poured into the container to cover the silicon nitride articles being sintered to protect them from thermal decomposition and reaction with the furnace materials. The setter powder-covered silicon nitride article is then placed in a batch-type furnace to sinter the silicon nitride. Without this protection, experience has shown that a silicon nitride article will decompose at temperatures above 1700.degree. C. in part to silicon and nitrogen. Furthermore, the silicon nitride article will react with the carbon in the graphite to form silicon carbide, which is thermodynamically stable at temperatures above 1700.degree. C., causing warping of the silicon nitride article. In contrast, when setter powder is used, the powder itself decomposes due to the high temperature in the furnace and the powder reacts preferentially with the furnace material instead of the silicon nitride article.
For economical reasons and optimum product quality, it is highly desirable to prepare sintered silicon nitride in a continuous process. Although silicon nitride could be sintered in continuous furnaces using setter powder in the above-described method, this method has been found to be undesirable. Firstly, the setter powder must be cleaned off the article after sintering, adding a step in the procedure. At times, the powder on a silicon nitride article sinters together or sinters to the article, thus, requiring excessive force to remove the sintered material. In some of these instances, the sintered setter powder cannot be removed even with force, and the article must be scrapped.
In addition, use of the setter powder is costly, as recycling of the setter powder is not without its problems. During use, there is inherent loss of setter powder (due to spillage, etc.). It has been known to lose as much powder in weight as the silicon nitride article itself. Furthermore, during recycling, often the setter powder needs to be crushed and screened to breakup the sintered pieces before reusing, adding yet another step in the manufacturing procedure.
Yttria or other densification agents are sometimes added to the setter powder, because, if the silicon nitride article contains these densification agents, the densification agents are "robbed" from the article by the setter powder during sintering unless the setter powder also contains the agents. In instances when densification agents are added to the setter powder, the setter powder typically is chemically analyzed to ensure that the required amounts of densification agents are present in the proper amounts. The analysis adds yet another step and additional complications to the manufacturing operation.
During sintering, the silicon nitride converts from a non-densified predominantly alpha-phase silicon nitride to a densified beta-phase silicon nitride. The present inventor has discovered that predominantly alpha-phase silicon nitride compositions free of densification aids can advantageously be converted to non-densified predominantly beta-phase silicon nitride by subjecting alpha-phase silicon nitride to sintering conditions.
It is also desirable to have available an improved silicon nitride material, one which is more consistent in physical and chemical properties throughout the bulk of the material and one which is high in strength.
It is, therefore, an object of the present invention to provide an economical method of converting alpha-phase silicon nitride to beta-phase silicon nitride which inhibits the silicon nitride from thermally decomposing or being adversely effected by furnace materials during conversion without using setter powder.
It is another object of the present invention to provide a method of converting alpha-phase silicon nitride to beta-phase silicon nitride which gives protective coverage to the silicon nitride during the conversion operation and which is easy to use, readily reusable, and which does not require any additional steps of operation.
It is yet another object of the present invention to provide a commercially-viable method for the continuous conversion of alpha-phase silicon nitride to beta-phase silicon nitride without experiencing the problems of using setter powder.
It is also another object of the present invention to provide a method for the continuous conversion of alpha-phase silicon nitride to beta-phase silicon nitride using conventional continuous furnaces.
It is yet another object of the present invention to provide a furnace for converting alpha-phase silicon nitride to beta-phase silicon nitride without the above-discussed problems.
It is another object of the present invention to provide an improved silicon nitride material which has more consistent properties throughout the bulk of the material and which has a high strength.